The invention relates to a current transformer comprising a closed magnetic circuit made up of stacked metal plates surrounding a primary conductor, and a secondary winding whose core is a part of the magnetic circuit.
Known current transformers, used notably in trip relays or circuit breakers, supply electrical power and current measuring signals to processing units. In state-of-the-art manner, the transformers comprise a primary winding formed by a power conductor through which a strong current flows, a magnetic circuit surrounding the power conductor and a secondary winding formed by one or two coils situated on the magnetic circuit.
The magnetic circuit of the transformers is constituted by stacked metal plates. To enable the coils of the secondary winding to be fitted, the plates preferably have open shapes. The plates are inserted inside the coils then fixed to one another so as to close the magnetic circuit.
Fixing of the stacked plates is performed by weldings which secure the stacks of plates end to end. With this fixing mode the magnetic circuit is liable to present large air-gaps due to the irregularities of the cut-outs of the plates and of the stacking. These air-gaps reduce the performances of the transformer at low current.
Current transformers exist comprising a magnetic circuit formed by closed and stacked plates not presenting any air-gap. In these transformers, the secondary winding has to be coiled directly onto the magnetic circuit. This manufacturing mode eliminates the risks of presence of air-gaps but considerably increases the complexity, manufacturing time and cost of the transformers.
The object of the invention is to achieve a current transformer having a high response at low current and which is simple and quick to manufacture.
This object is achieved by the fact that each plate of the magnetic circuit is formed by a single part surrounding the primary conductor and being severed to define a gap or cut-out at a single location, the plates being stacked in the same position, the cut-out of each plate coinciding with the cut-outs of the other plates.
In a preferred embodiment, the magnetic circuit comprises a fixed first part designed to receive the secondary winding and a flexible second part able to be momentarily deformed.
The transformer may comprise securing means to hold the plates in the plane of their cutting.
The plate cut-outs are preferably located in a part of the magnetic circuit prolonging the core of the secondary winding, the plates being, for example, cut perpendicularly to the axis of the secondary winding or parallel to the axis of the secondary winding.
According to a particular embodiment the magnetic circuit is rectangular in shape and comprises a first large side receiving the secondary winding, a second large side opposite the first large side, and two small sides, the cut-outs of the magnetic circuit being situated between the first large side and an adjacent small side. The fixed first part of the magnetic circuit comprises the first large side and the flexible second part comprises at least the opposite large side or the adjacent small side. In this embodiment, the cut-outs are preferably made according to a line bisecting a right angle formed by the first large side and an adjacent small side.
In a manufacturing process of a transformer according to the invention, the metal plates are cut at a single place and stacked making the cut-outs of each plate correspond, then, in a first stage a flexible part of the plates of the magnetic circuit is deformed, in a second stage a secondary winding coil is arranged on a fixed part of the magnetic circuit, and in a third stage the flexible part of the plates of the magnetic circuit is put back in the normal position, the fixed and flexible parts of each plate being aligned according to their cutting plane.